A variety of physical and chemical deposition procedures have been used to prepare aluminum (Al) films. These methods are of interest, in part, because thin films of aluminum have many uses due to their high electrical conductivity, high reflectivity, mechanical strength, and their resistance to chemical attack. There is much current interest in generating thin films of aluminum using chemical vapor deposition (CVD), particularly resulting from applications in the microelectronics industry. In a typical CVD process, organoaluminum precursors are volatilized and then decomposed to yield aluminum, which is deposited as a film on the target substrate.
A series of stable, volatile donor-acceptor complexes of alane (AlH.sub.3) have been known for many years. They can be generally represented by D.AlH.sub.3, and can be readily synthesized in one step from LiAlH.sub.4. These donor-acceptor complexes of alane are air sensitive, but they are not pyrophoric, as are the trialkylaluminums. Among the known donors (D) are Me.sub.3 N, Et.sub.3 N, Me.sub.3 P, Me.sub.2 S, and tetrahydrofuran (THF). See, for example, E. Wiberg et al., Hydrides of the Elements of Main Groups I-IV, Elsevier: Amsterdam, Ch. 5 (1971); and R. A. Kovar et al., Inorg. Synth., 17, 36 (1977). Trimethylamine is unique among these donors in its ability to form a bis complex with alane, i.e., (Me.sub.3 N).sub.2 AlH.sub.3.
The use of amine-alane complexes for the vapor phase deposition of aluminum have been disclosed by T. P. Whaley et al., U.S. Pat. No. 3,206,326 (1965), and in D. R. Carley et al. U.S. Pat. No. 3,375,124 (1968). These methods, however, do not produce mirror-like coatings. Rather, less reflective "shiny" and "metallic" surfaces result. Laser-induced deposition of aluminum using aminealane complexes has been disclosed in T. H. Baum et al., Abstracts of Papers, Fall Meeting, Boston, Mass.; Materials Research Society: Pittsburgh, Pa., B4.12 (1988).
Trimethylamine alane (TMAA) has emerged as a promising source for aluminum in the growth of metallic aluminum films and thin films of aluminum gallium arsenide. For example, see W. L. Gladfelter et al. (U.S. Pat. No. 4,923,717); W. L. Gladfelter et al., Chem. Mater., 1, 339 (1989); and E. R. Abernathy et al., Appl. Phys. Lett., 56, 2654 (1990). For the growth of aluminum thin films, its advantage over triisobutylaluminum includes higher growth rates and lower required growth temperatures. In addition, it is not pyrophoric. Aluminum gallium arsenide films grown with TMAA have exhibited appreciably lower carbon and oxygen contents when compared to films grown from aluminum precursors that have direct Al-C bonds, such as triethylaluminum. (J. S. Roberts et al., J. Crystal Growth, 104, 857 (1990)). Despite its relatively high volatility (vapor pressure at room temperature=1.8 torr), a technological disadvantage of TMAA is that it is a solid. Thus, it is difficult to deliver it at a uniform flow rate to the substrate.
Alternative tertiaryamine complexes of alane are known, and several of these, such as triethylamine alane, tri-n-propylamine alane, and tri-n-butylamine alane, are liquids. See, J. K. Ruff et al., J. Amer. Chem. Soc., 82, 2141 (1960). Unfortunately, the thermal stability of the donor-acceptor complex decreases as the stearic bulk of the donor increases. Thus, TMAA is the most stable and tri-n-butylamine is the least stable at room temperature. Triethylamine alane has been reported to give high quality aluminum films, but it is substantially less stable than TMAA See, L. H. Dubois et al., Surf. Sci., 244, 89 (1991).
Therefore, a need exists for amine alane precursors for aluminum films which are both sufficiently stable and volatile, so that they can be used to provide high quality aluminum films on a variety of substrates.